(The formation,
indeed, of a large crystal of any mineral in a rock of mixed composition
implies an aggregation of the requisite atoms, allied to concretionary
action. The cause of the crystals of feldspar in these rocks of Ascension,
being all placed lengthways, is probably the same with that which elongates
and flattens all the brown sphaerulitic globules (which behave like
feldspar under the blowpipe) in this same direction.) These allied forces,
therefore, have played an important part in the lamination of the mass, but
they cannot be considered the primary force; for the several kinds of
nodules, both the smallest and largest, are internally zoned with
excessively fine shades of colour, parallel to the lamination of the whole;
and many of them are, also, externally marked in the same direction with
parallel ridges and furrows, which have not been produced by weathering.

Some of the finest streaks of colour in the stony layers, alternating with
the obsidian, can be distinctly seen to be due to an incipient
crystallisation of the constituent minerals. The extent to which the
minerals have crystallised can, also, be distinctly seen to be connected
with the greater or less size, and with the number, of the minute,
flattened, crenulated air-cavities or fissures. Numerous facts, as in the
case of geodes, and of cavities in silicified wood, in primary rocks, and
in veins, show that crystallisation is much favoured by space. Hence, I
conclude, that, if in a mass of cooling volcanic rock, any cause produced
in parallel planes a number of minute fissures or zones of less tension
(which from the pent-up vapours would often be expanded into crenulated
air-cavities), the crystallisation of the constituent parts, and probably
the formation of concretions, would be superinduced or much favoured in
such planes; and thus, a laminated structure of the kind we are here
considering would be generated.

That some cause does produce parallel zones of less tension in volcanic
rocks, during their consolidation, we must admit in the case of the thin
alternate layers of obsidian and pumice described by Humboldt, and of the
small, flattened, crenulated air-cells in the laminated rocks of Ascension;
for on no other principle can we conceive why the confined vapours should
through their expansion form air-cells or fibres in separate, parallel
planes, instead of irregularly throughout the mass. In Mr. Stokes'
collection, I have seen a beautiful example of this structure, in a
specimen of obsidian from Mexico, which is shaded and zoned, like the
finest agate, with numerous, straight, parallel layers, more or less opaque
and white, or almost perfectly glassy; the degree of opacity and glassiness
depending on the number of microscopically minute, flattened air-cells; in
this case, it is scarcely possible to doubt but that the mass, to which the
fragment belonged, must have been subjected to some, probably prolonged,
action, causing the tension slightly to vary in the successive planes.

Several causes appear capable of producing zones of different tension, in
masses semi-liquified by heat. In a fragment of devitrified glass, I have
observed layers of sphaerulites which appeared, from the manner in which
they were abruptly bent, to have been produced by the simple contraction of
the mass in the vessel, in which it cooled. In certain dikes on Mount Etna,
described by M. Elie de Beaumont ("Mem. pour servir" etc. tome 4 page
131.), as bordered by alternating bands of scoriaceous and compact rock,
one is led to suppose that the stretching movement of the surrounding
strata, which originally produced the fissures, continued whilst the
injected rock remained fluid. Guided, however, by Professor Forbes'
("Edinburgh New Phil. Journal" 1842 page 350.) clear description of the
zoned structure of glacier-ice, far the most probable explanation of the
laminated structure of these feldspathic rocks appears to be, that they
have been stretched whilst slowly flowing onwards in a pasty condition (I
presume that this is nearly the same explanation which Mr.